The present invention relates to a charging apparatus for charging an image bearing member used in an image forming apparatus, such as a copying machine or a printer, which employs an electrophotographic system, an electrostatic recording system, or the like. In particular, it relates to a charging apparatus which employs a magnetic brush based charging system.
An image forming apparatus such as a copying machine, a page printer, or the like, which employs an electrophotographic system, an electrostatic recording system, or the like, also employs a charging apparatus for charging (inclusive of removing electrical charge) a latent image bearing member, for example, an electrophotographic photosensitive member, an electrostatically recordable dielectric member, or the like, or objects different from a latent image bearing member. In the past a corona based charging device, which is a noncontact device, has been used as the above described charging apparatus for an image forming apparatus.
In recent years, a contact charging apparatus has been put to practical use, and has been replacing a corona based charging device. This is due to the fact that a contact charging apparatus is low in ozone production and power consumption, compared to a noncontact charging apparatus. There are various contact charging apparatuses; those which employ a roller based charging system, that is, those which employ an electrically conductive roller as a contact charging member, have been preferably used because of their stability in charging performance. In a roller based charging system, an electrically conductive, elastic roller as a contact charging member is directly pressed upon an object (hereinafter, xe2x80x9cphotosensitive memberxe2x80x9d) which is to be charged, and the photosensitive member is charged by applying voltage to the roller.
However, even in the case of a contact charging apparatus, its fundamental charging mechanism is such that a photosensitive member is charged through electrical discharge from a contact charging member to the photosensitive member. Therefore, the value of the voltage applied to a contact charging member in order to charge a photosensitive member to a desired surface potential level must be greater than the value of the desired surface potential level. Also, ozone is generated although the amount is small. Further, when AC voltage is included as a component of the voltage applied to a contact charging member in order to improve the uniformity with which a photosensitive member is charged, unprecedented problems occur. For example, the amount by which ozone is generated increases; a contact charging member and a photosensitive member are caused to vibrate by the electrical field generated by the AC voltage, resulting in noises; and the rate at which the surface of a photosensitive member is deteriorated by electrical discharge is accelerated.
Thus, there has been desired a method for charging a photosensitive member by directly injecting electrical charge into the photosensitive member. U.S. Pat. No. 5,606,401 discloses such a charging method in which the peripheral surface of a photosensitive member is provided with a charge injection layer, and electrical charge is injected into this layer by a contact charging member. In this method, in other words, with the provision of a charge injection layer, a photosensitive member is sufficiently charged in a short time even if a contact charging member, the resistance value of which is no less than 1xc3x97404 xcexa9, is used as a contact charging member, making it possible to solve the above described problems of a contact charging system, that is, the problems traceable to electrical discharge, at their roots.
More specifically, a magnetic brush type charging member which comprises a magnetic brush portion formed by magnetically confining electrically conductive magnetic particle in the form of a brush is employed. In charging a photosensitive member, this magnetic brush portion is placed in contact with the photosensitive member. A magnetic type charging member is preferably used because it makes it possible to create a larger contact nip between the charging member and a photosensitive member, and also because it can be placed evenly in contact with the peripheral surface of a photosensitive member, in terms of a microscopic level, to prevent the photosensitive member from failing to be properly charged.
The magnetic brush portion of the above described magnetic brush type charging member is formed by magnetically confining particles of magnetic material (hereinafter, they may be referred to as xe2x80x9ccharge carrierxe2x80x9d), on the surface of a magnetic particle bearing member. More specifically, magnetic particles, such as ferrite particles, the resistance value of which is in a range of 1xc3x97104-1xc3x97108 xcexa9xc2x7cm, are magnetically confined in the form of a brush directly on a magnet, or on the peripheral surface of a sleeve in which the magnet is contained. In order to charge a photosensitive member, the magnetic brush type charging member is rotated, with its magnetic brush portion being placed in contact with the photosensitive member, and voltage is applied to the charging member.
One of the factors that affect the charging performance of a magnetic brush type charging apparatus which employs a magnetic brush, such as the one described above, as a contact charging member, is degree of uniformity with which the magnetic brush is placed in contact with the peripheral surface of a photosensitive member. In order to place the magnetic brush portion uniformly across the peripheral surface of a photosensitive member, a magnetic brush type charging apparatus is desired to be equipped with a regulating means for regulating the thickness of the magnetic brush, i.e., the layer of magnetic particles, on a magnetic particle bearing member, on the upstream side in terms of the rotational direction of the magnetic particle bearing member. With the provision of such a means, it is possible to accomplish uniformity in charging a photosensitive member. In other words, the provision of such a means makes it possible to provide a high degree of stability in charging a photosensitive member, and therefore, is preferable.
As for the material for the charge injection layer of a photosensitive member, compound material composed by dispersing microscopic electrically conductive particles in electrically insulative and transparent binder is preferably used. While the magnetic brush portion, to which voltage is being applied, is placed in contact with the charge injection layer, the electrically conductive particles behave as if they were numerous independent floating electrodes. As a result, numerous virtual condensers are formed by the electrically conductive substrate of the photosensitive member and these numerous floating electrodes, i.e., the electrically conductive particles, and electrical charge is taken up by these virtual condensers.
Therefore, the voltage applied to the contact charging member and the surface potential level of the photosensitive member converge to approximately the same value. In other words, the employment of a magnetic brush type charging member makes it possible to realize a low voltage charging method.
A charging method such as the one described above (method for charging an object by directly injecting electrical charge into the object) is called xe2x80x9ccharge injectionxe2x80x9d. With the use of a charge injection apparatus (charge injection device), it is possible to realize a cleanerless image forming apparatus, that is, a transfer type image forming apparatus, which does not require a cleaner dedicated for cleaning the toner particles left on the image forming apparatus after image transfer.
b) Cleanerless System.
Next, a cleanerless system will be described. A cleanerless system is a cleaning system employed by a transfer type image forming apparatus which employs the so-called reversal developing system. In the reversal developing system, a photosensitive member is negatively charged, and a latent image is developed by adhering negatively charged toner to the exposed portions, that is, the portions with a reduced potential level.
In a cleanerless system, of the smaller amount of transfer residual toner particles, that is, toner particles remaining on a photosensitive member after image transfer, those with positive polarity are temporarily taken in by a charge injection apparatus. Then, they are charged to negative polarity in the charge injection apparatus, and then, are ejected back onto the photosensitive member from the charge injection apparatus. On the other hand, the transfer residual toner particles, the polarity of which remained negative, are mostly not taken in by the charge injection apparatus and are recovered, along with the transfer residual toner particles, which have been ejected from the charging injection apparatus, by a developing apparatus through a process for developing a latent image.
More specifically, the transfer residual toner particles are taken into a developing apparatus by the fog removal bias, that is, difference Vback in potential level between the DC voltage applied to the developing apparatus, and the surface potential level of the photosensitive member, during a developing process.
According to this method, a portion of the transfer residual toner particles is recovered by the developing apparatus, by way of the charge injection apparatus, and the rest is directly recovered by the developing apparatus. After the recovery, they all are used in the following development processes. Therefore, no waste toner is generated, reducing the need for troublesome maintenance. Being cleanerless has merits in terms of space; the elimination of a cleaning apparatus drastically reduces the size of an image forming apparatus.
From the standpoint of the efficiency with which the transfer residual toner particles are recovered during a developing process, it is desired that an image forming apparatus employs a developing apparatus structured to place developer, for example, toner, or mixture of toner and carrier, in contact with a photosensitive member. In other words, it is desired that an image forming apparatus employs a developing apparatus which employs a contact developing system which uses single or two component developer.
However, a contact charging apparatus has its own problem. That is, in a contact charging apparatus, the contact charging member placed in contact with an object to be charged picks up the contaminants on the object to be charged, or other foreign matter, becoming substantially contaminated with the increase in accumulated usage, which results in decrease in charging performance. This is also true with a magnetic brush type charging apparatus.
Some image forming apparatuses with a magnetic brush type charging apparatus are provided with a cleaner dedicated for removing the transfer residual toner particles after image transfer. Even in the cases of such image forming apparatuses, toner particles or the like, tend to slip through the cleaner, although by only a small amount, and are carried to the position of the magnetic brush portion (charging nip) by the rotation of the photosensitive member, thus adhering to, or mixing into, the magnetic brush portion. Eventually, the transfer residual toner and the like, which passed through the cleaner, contaminate the magnetic brush portion, as an image formation cycle is repeated.
Normally, toner particles relatively high in electrical resistance are used as the toner particles for a developing apparatus. Therefore, as a relatively large amount of toner particles adheres to, or mixes into, the magnetic brush portion, in other words, as an excessive amount of toner particles accumulates around, or in, the magnetic brush portion, the electrical resistance of the magnetic brush type charging member, as a contact charging member, increases throughout the entirety, or some parts, of the charging member, resulting sometimes in a situation in which a photosensitive member fails to be charged to a desired potential level, and/or it is unevenly charged.
The above described type of contamination of a magnetic brush type charging member by toner particles, and resultant images of poor quality, are conspicuous, particularly, in the case of a cleanerless image forming apparatus, that is, an image forming apparatus which is not equipped with a cleaner dedicated for removing the transfer residual toner on a photosensitive member after image transfer.
More specifically, in the case of an image forming apparatus which employs a cleanerless system, a relatively large amount of the toner particles, which remains on a photosensitive member after image transfer, is carried, as it is, to the charging nip, in which it adheres, or mixed into, the magnetic brush portion on the magnetic brush bearing portion of a magnetic brush type charging member, tending to cause the magnetic brush type charging member to become excessively contaminated at a relatively early stage of its usage.
Further, in the case of a cleanerless system, the positively charged transfer residual toner particles on a photosensitive drum are temporarily taken into a charge injection device, in which their polarity is reversed. Thereafter, they are ejected back onto the photosensitive member. However, as a high density image is repeatedly and continuously formed across a particular area on a photosensitive member, the adhesion of toner particles tends to concentrate to the portion of the magnetic brush portion corresponding to this particular area on the photosensitive member. In other words, as a high density image is formed across a particular area on a photosensitive image for an extended length of time, the value of the electrical resistance of the magnetic brush portion increases, reducing the charging ability of the charging member, only across the portion of the magnetic brush portion corresponding to the area on the photosensitive member, across which the high density image is formed, whereas other portions of the magnetic brush portion retain a sufficient amount of charging ability. As a result, the photosensitive member is unevenly charged, which results in images irregular in density.
It may be assumed that if the charge carrier particles which form a magnetic brush shift or circulate in a charging apparatus, local contaminants of the magnetic brush are dispersed, being therefore diluted with the elapse of time. However, if the amount of the charge carrier placed in a charging apparatus scarcely exceeds the proper amount of the charge carrier to be borne on the peripheral surface of a sleeve, as a magnetic brush bearing member, to satisfactorily form a magnetic brush, the charge carrier hardly disperses in the longitudinal direction of the sleeve.
When the amount by which charge carrier is placed in the charging apparatus is increased so that a certain amount of charge carrier will remain in the charging apparatus after the charge carrier in the charging apparatus is borne on the peripheral surface of the sleeve by a proper amount, the charge carrier tends to slightly disperse with the elapse of time. However, the amount by which the charge carrier disperses in this case is not sufficient to eliminate the above described problem. This is due to the fact that charge carrier is very large in powder density, and therefore, hardly moves either straight or circulatorily within a charging apparatus.
Thus, an idea of providing a charging apparatus with a charge carrier stirring apparatus to circulate the charge carrier within the charging apparatus has been studied. However, it has been extremely difficult to effectively circulate charge carrier, since charge carrier is high in powder density as described above.
The primary object of the present invention is to provide a charging apparatus, the magnetic brush of which is prevented from being locally contaminated.
Another object of the present invention is to provide a charging apparatus which is excellent in the ability to circulate magnetic particles.
According to one of the aspects of the present invention, a charging apparatus comprises: magnetic particles which charge an object by being rubbed against the object; a particle bearing member which rotates while magnetically bearing the magnetic particles; and a stirring member disposed above said particle bearing member to stir the magnetic particles, wherein said stirring member moves in the direction opposite to the moving direction of said particle bearing member, in the area in which the distance between said stirring member and particle bearing member is smallest.